This invention relates generally to hybrid powertrains for motor vehicles having two power sources or prime movers, such as an electric motor and internal combustion engine. In particular, the invention pertains to a powertrain that provides the driver with an acceptable response to a command for a change in wheel torque even though the prime mover is slow to respond to the command.
Hybrid electric powertrains for motor vehicles include at least two power sources or prime movers, an electric motor or motor/generator, accessible to an electric storage unit such as a battery, and an engine, such as an internal combustion engine supplied from a fuel source. Powertrains for another type of motor vehicle, electric drive, include one power source, an electric motor or motor/generator supplied from an electric storage battery.
The electric machine systems for either type of powertrain may use a battery, flywheel, capacitor, fuel cell, or other energy storage devices. The engines in hybrid powertrains may be gasoline, diesel, or other fuel-based combustion systems.
The rotational speed and torque states of a hybrid electric powertrain having either a step ratio transmission or fixed-span continuously variable transmission cannot be transiently isolated from the speed and torque states of the driveline and wheels demanded by the operator. This lack of isolation imposes constraints on the efficient use of hybrid electric, kinetic energy storage systems.
Certain types of internal combustion engines and fuel cell systems can provide high efficiency and low emissions, but with relatively slow transient load and speed response. This slow transient response is unacceptable for vehicles with conventional drivelines.
Certain types of transmissions provide acceptable vehicle transient response even when used in combination with slow transient response engines. The basic features of these transmissions include an infinitely variable drive ratio, and reversible torque machines with energy storage. An infinitely variable transmission (IVT) is a transmission that produces an infinite speed ratio span and continuous speed ratio variation.
Internal combustion engine development has been concentrated on efficient power sources such as engines and combustion systems that produce low exhaust gas emissions. Gasoline engines that use homogeneous charge compression ignition (HCCI) combustion show potential to operate as efficiently as a diesel engine, yet emit a very low volume of NOx compounds in the exhaust gas. However, on a fixed compression ratio basis, a HCCI combustion system is five to ten times slower in response to load changes than a stoichiometric, port fuel injected, spark ignited combustion system, the current production baseline engine.
Slow dynamic engine response requires both speed isolation and load isolation to produce acceptable performance. The key powertrains for producing speed and load isolation include series electric, series hybrid, power-split electric and power-split hydraulic. The prime mover would be suitably decoupled from the drive wheels, as in most conventional high power hybrid powertrains.
An energy storage media onboard the vehicle supplements energy derived from engine fuel. The energy storage media must be capable of powering the vehicle for the length of the period while the prime mover is adjusting to the new load state, generally about one to five seconds. Both hydraulic and electric hybrid architectures may be used to provide the auxiliary energy storage.
Energy stored in hydraulic fluid accumulators, electric batteries, capacitors, or flywheels can be used in combination with reversible machines, such as hydraulic pump/motors and electric motors, to accelerate the vehicle in response to the driver's demand for increased wheel torque, while the prime mover is moving from an idle state to a full load state. The stored energy reserve can be recovered from regenerative wheel braking or it can be produced by the prime mover while road load is low.
There is a need to provide a powertrain having components that compensate for the inherently slow dynamic response of a prime mover such as an IC engine or other power source to provide the operator with an acceptable, timely response to speed and load transient demands.